In Limulus striated muscle, thick filaments shorten as the sarcomere shortens below rest length. Length tension studies show that this filament shortening is tension generating. Here we propose to investigate possible mechanisms involved in the filament shortening process. 1. Myosin ATPase activity of isolated long and short thick filaments will be determined to show whether shortening reduces specific activity or not. 2. Thick filament proteins will be analyzed by high resolution gel electrophoresis (isoelectric focusing, first dimension and SDS gel electrophoresis second dimension). During shortening phosphorylation may occur and filament protein net charge should change. 3. Phosphorylation of filament proteins will be determined by using radiolabelled ATP and two-dimensional gel electrophoresis. 4. Net charge of long and short filaments will be determined by quantitating Na ion and K ion using proton induced x-ray emission spectroscopy at Brookhaven National Laboratory. 5. Total mass of isolated long and short filaments will be determined by scanning electron microscopy at Brookhaven National Laboratory.